High speed electron tunneling devices

ABSTRACT

A detector includes a voltage source for providing a bias voltage and first and second non-insulating layers, which are spaced apart such that the bias voltage can be applied therebetween and form an antenna for receiving electromagnetic radiation and directing it to a specific location within the detector. The detector also includes an arrangement serving as a transport of electrons, including tunneling, between and to the first and second non-insulating layers when electromagnetic radiation is received at the antenna. The arrangement includes a first insulating layer and a second layer configured such that using only the first insulating in the arrangement would result in a given value of nonlinearity in the transport of electrons while the inclusion of the second layer increases the nonlinearity above the given value. A portion of the electromagnetic radiation incident on the antenna is converted to an electrical signal at an output.

RELATED APPLICATION

The present application is a Continuation of U.S. patent applicationSer. No. 10/877,874, filed on Jun. 26, 2004; which is a Continuation ofU.S. patent application Ser. No. 10/347,534, filed on Jan. 20, 2003 andissued as U.S. Pat. No. 6,756,649 on Jun. 29, 2004; which is aContinuation of U.S. patent application Ser. No. 09/860,972, filed onMay 21, 2001 and issued as U.S. Pat. No. 6,563,185 on May 13, 2003; allof which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to optical devices and, moreparticularly, to optoelectronic devices based on electron tunneling.

The increasing speed of optical communications is fueling the race toachieve ever faster optical communications devices for transmitting,modulating and detecting electromagnetic signals. Terahertz speeds areexpected in the near future, and optical communication devices that canoperate at such high speeds are in great demand.

A possible approach to achieving high speed optoelectronic devices foruse as optical communication devices is electron tunneling. Electrontunneling-based devices, such as metal-insulator-metal (M-I-M) devicesfor use as infrared and far-infrared detectors and frequency mixers havebeen explored in the past (see, for example, S. M. Faris, et al.,“Detection of optical and infrared radiation with DC-biasedelectron-tunneling metal-barrier-metal diodes,” IEEE Journal of QuantumElectronics, vol. QE-9, no. 7 (1973); L. O. Hocker, et al., “Frequencymixing in the infrared and far-infrared using a metal-to-metal pointcontact diode,” Applied Physics Letters, vol. 12, no. 12 (1968); and C.Fumeaux, et al., “Nanometer thin-film Ni—NiO—Ni diodes for detection andmixing of 30 THz radiation,” Infrared Physics and Technology, 39(1998)). Such M-I-M devices generally operate on the basis of electronrectification and current production due to incident electromagneticenergy and resulting electron tunneling effects. M-I-M devices cannormally be used to rectify extremely high frequencies, extending intothe optical frequency range.

In addition to high frequency rectification, it is also desirable toachieve high degrees of asymmetry and nonlinearity in thecurrent-versus-voltage (I-V) curve in electron tunneling devices. Thedifferential resistance of the device, which corresponds to thesensitivity of the device to incoming electromagnetic energy, isdirectly related to the nonlinearity of the I-V curve. However, priorart M-I-M devices generally exhibit low degrees of asymmetry andnonlinearity in the electron transport such that the efficiency of suchdevices is limited. A high degree of nonlinearity improves the quantumefficiency of the electron tunneling device, which is number ofelectrons collected for each photon incident on the M-I-M device. Highquantum efficiency is crucial for efficient operation and highsensitivity of the M-I-M diode in all optoelectronic applications. Forpurposes of this application, a diode is defined as a two-terminaldevice. A high degree of nonlinearity offers specific advantages incertain applications. For example, in optical mixing, second orderderivatives of the current-voltage relationship determine the magnitudeof the signal produced in frequency down-conversion. A higher degree ofasymmetry in the I-V curve between positive values of V (forward biasvoltage) and negative values of V (reverse bias voltage) results inbetter rectification performance of the device. A high degree ofasymmetry is required, for example, to achieve efficient large signalrectification such as in the detection of high intensity incidentfields. In this high intensity field regime, the electron tunnelingdevice functions as a switch, and it is desirable to attain a lowresistance value in one polarity of voltage and a high resistance valuein the opposite polarity of voltage is desired. Alternatively, with lowfield intensities and large photon energies, the incident field sweeps alarger portion of the electron tunneling device dark I-V curve and,consequently, the high asymmetry translates into high responsivity andas well as high quantum efficiency and sensitivity in electromagneticradiation detection.

The fabrication of the combinations of alternate layers of metals andinsulators in M-I-M-based devices, in comparison to semiconductormaterials, is advantageous due to ease of deposition of materialsrelative to semiconductor fabrication. It has been suggested that therecent trend of decreasing the size of electronic devices to achievehigh speed switching will ultimately make semiconductor-based devicesimpractical due to fluctuation of carrier concentration, which occurswhen semiconductor devices are reduced to mesoscopic regimes (see, forexample, Suemasu, et al., “Metal (CoSi₂)/Insulator(CaF₂) resonanttunneling diode,” Japanese Journal of Applied Physics, vol. 33 (1994),hereafter Suemasu). Moreover, semiconductor devices are generally singlebandgap energy devices. This characteristic of semiconductor devicesmeans that, in detection applications, no current is produced when aphoton having energy less than the bandgap energy is incident on thesemiconductor device. In other words, the response of the semiconductordevice is limited by the bandgap energy. When a semiconductor diode isused to rectify high frequency oscillations, the semiconductor materiallimits the frequency response of the diode because the charge carriersmust be transported through a band, in which concentration are limitedin comparison to a metal.

Existing electron tunneling devices based on metal-oxide combinationsare generally fabricated by forming a metal layer, exposing the metallayer for a certain amount of time such that the native oxide of themetal is formed, then repeating the process as desired. Photolithographytechniques may also be used to achieve desired shapes and patterns inthe metals and insulators. For example, Suemasu describes a metal(CoSi₂)/insulator(CaF₂) resonant tunneling diode with a configurationM-I-M-I-M-I-M triple-barrier structure for use as long wavelength(far-infrared and milliwave) detectors and emitters. However, theM-I-M-I-M-I-M device of Suemasu is much more complicated than the simpleM-I-M devices, and must be fabricated using a complex epitaxial growthprocedure using exotic materials. In fact, Suemasu chooses to use thetriple-barrier structure rather than a slightly simpler double-barrierstructure for apparently better performance results in the electrontunneling process. Therefore, although the M-I-M-I-M-I-M device ofSuemasu achieves much higher degrees of asymmetry and nonlinearity inthe I-V curve than the M-I-M devices, the performance gains come at thecost of the simplicity in design and fabrication.

An alternative approach is the use of a combination of a metal and asemiconductor in a metal-insulator-semiconductor (MIS) configuration(see, for example, T. Yamada, et al., “Semiconductor Device Using MISCapacitor,” U.S. Pat. No. 5,018,000, issued 21 May 1991). The drawbackto currently available MIS devices is also the limited efficiency due toasymmetry and nonlinearity limitations. MIS devices cannot operate at ashigh frequencies as M-I-M devices because the concentration of electronstates in the semiconductor is lower than that from a metal.

At this time, infrared detectors, for example, capable of receivingelectromagnetic signal at terahertz rates, at room temperature, are notreadily available, to the Applicants' knowledge. Temperature-controlledalternatives, such as narrow bandgap semiconductor detectors, andbolometers, exist on the market, but the extra considerations associatedwith the temperature control mechanism make such devices expensive andbulky. Prior art M-I-M detectors are capable of detecting infraredradiation without cooling, but these prior art detectors are notsensitive enough for practical applications.

As will be seen hereinafter, the present invention provides asignificant improvement over the prior art as discussed above by virtueof its ability to provide the increased performance while, at the sametime, having significant advantages in its manufacturability.

SUMMARY

As will be described in more detail hereinafter, a number of high speedelectron tunneling devices are disclosed herein. The devices of thepresent invention are especially distinguishable from the aforementionedelectron tunneling devices of the prior art by the implementation ofresonant tunneling using at least one layer of an amorphous material inthe devices. In a first aspect of the invention, a detector fordetecting electromagnetic radiation incident thereon is disclosed. Thedetector has an output, exhibits a given responsivity and includes firstand second non-insulating layers spaced apart from one another such thata given voltage can be applied across the first and secondnon-insulating layers, the first non-insulating layer being formed of ametal, and the first and second non-insulating layers being configuredto form an antenna structure for receiving electromagnetic radiation.The detector further includes an arrangement disposed between the firstand second non-insulating layers and configured to serve as a transportof electrons between the first and second non-insulating layers as aresult of the electromagnetic radiation being received at the antennastructure. This arrangement includes a first layer of an amorphousmaterial and a second layer of material, configured to cooperate withthe first layer of the amorphous material such that the transport ofelectrons includes, at least in part, transport by tunneling, and suchthat at least a portion of the electromagnetic radiation incident on theantenna is converted to an electrical signal at the output, theelectrical signal having an intensity which depends on the givenresponsivity. For purposes of this application, an amorphous material isconsidered to include all materials which are not single crystal instructure.

In a second aspect of the invention, an emitter for providingelectromagnetic radiation of a desired frequency at an output isdescribed. The emitter includes a voltage source, for providing a biasvoltage, and first and second non-insulating layers, whichnon-insulating layers are spaced apart from one another such that thebias voltage can be applied across the first and second non-insulatinglayers. The emitter also includes an arrangement disposed between thefirst and second non-insulating layers and configured to serve as atransport of electrons between the first and second non-insulatinglayers as a result of the bias voltage. This arrangement is configuredto exhibit a negative differential resistance when the bias voltage isapplied across the first and second non-insulating layers. Thearrangement includes a first layer of an amorphous material and a secondlayer of material, which second layer of material is configured tocooperate with the first layer of amorphous material such that thetransport of electrons includes, at least in part, transport by means oftunneling, and such that an oscillation in the transport of electronsresults. This oscillation has an oscillation frequency equal to thedesired frequency, due to the negative differential resistance, andcauses an emission of electromagnetic radiation of the desired frequencyat the output.

In a third aspect of the invention, a modulator for modulating an inputelectromagnetic radiation incident thereon and providing a modulatedelectromagnetic radiation at an output is described. The modulatorincludes a voltage source for providing a modulation voltage, whichmodulation voltage is switchable between first and second voltagevalues. The modulator also includes first and second non-insulatinglayers spaced apart from one another such that the modulation voltagecan be applied across the first and second non-insulating layers. Thefirst and second non-insulating layers are configured to form an antennastructure for absorbing a given fraction of the input electromagneticradiation with a given value of absorptivity, while a remainder of theinput electromagnetic radiation is reflected by the antenna structure,where absorptivity is defined as a ratio of an intensity of the givenfraction to a total intensity of the input electromagnetic radiation.The modulator further includes an arrangement disposed between the firstand second non-insulating layers and configured to serve as a transportof electrons between the first and second non-insulating layers as aresult of the modulation voltage. This arrangement includes a firstlayer of an amorphous material and a second layer of material, whichsecond layer of material is configured to cooperate with the first layerof the amorphous material such that the transport of electrons includes,at least in part, transport by means of tunneling, with respect to themodulation voltage. The arrangement is configured to cooperate with thefirst and second non-insulating layers such that the antenna exhibits afirst value of absorptivity, when modulation voltage of the firstvoltage value is applied across the first and second non-insulatinglayers, and exhibits a distinct, second value of absorptivity, whenmodulation voltage of the second voltage value is applied across thefirst and second non-insulating layers, causing the antenna structure toreflect a different amount of the input electromagnetic radiation to theoutput as modulated electromagnetic radiation, depending on themodulation voltage. The modulator is configurable to operate as adigital device, in which only discrete, first and second voltage valuesare used, or as an analog device, in which continuous values of voltagebetween the aforedescribed first and second voltage values are used toachieve a continuum of values of absorptivity between the aforementionedfirst and second values of absorptivity.

In a fourth aspect of the present invention, a modulator assembly forreceiving a modulation signal, modulating an input electromagneticradiation and providing an output electromagnetic radiation isdescribed. The modulator assembly includes a voltage source forproviding a bias voltage, and first and second non-insulating layers,which non-insulating layers are spaced apart from one another such thatthe bias voltage can be applied across the first and secondnon-insulating layers. The first and second non-insulating layers arealso configured to form a first antenna structure for receiving themodulation signal and converting the modulation signal so received intoa modulation voltage, which modulation voltage is also applied acrossthe first and second non-insulating layers. The modulator assembly alsoincludes an arrangement disposed between the first and secondnon-insulating layers and configured to serve as a transport ofelectrons between the first and second non-insulating layers as a resultof the modulation voltage. The arrangement includes a first layer of anamorphous material and a second layer of material, which second layer ofmaterial is configured to cooperate with the first layer of theamorphous material such that the transport of electrons includes, atleast in part, transport by means of tunneling. The modulator assemblyfurther includes a second antenna structure having an absorptance value,which absorptance value depends on the aforementioned modulationvoltage. The second antenna structure is configured to receive andselectively absorb the input electromagnetic radiation in proportion tothe absorptance value so as to produce the output electromagneticradiation.

In an fifth aspect of the present invention, a field effect transistorfor receiving an external signal, switching an input signal according tothe received, external signal and providing an output signal isdescribed. The external signal is switchable between a first value and asecond value, and the field effect transistor includes a diode structureincluding a source electrode for receiving the input signal and a drainelectrode spaced apart from the source electrode such that a givenvoltage can be applied across the source and drain electrodes. The diodestructure further includes an arrangement disposed between the sourceand drain electrodes and configured to serve as a transport of electronsbetween the source and drain electrodes. The arrangement includes atleast a first layer of an amorphous material configured such that thetransport of electrons includes, at least in part, transport by means ofresonant tunneling with a given value of a tunneling probability. Thefield effect transistor also includes a shielding layer at leastpartially surrounding the diode structure. The field effect transistorfurther includes a gate electrode disposed adjacent to the shieldinglayer and is configured to receive the external signal and to apply theexternal signal as the given voltage across the source and drainelectrodes such that, when the first value of external signal isreceived at the gate electrode, a first signal value is provided as theoutput signal at the drain electrode and, when the second value ofexternal signal is received at the gate electrode, a second signal isprovided as the output signal at the drain electrode.

In a sixth aspect of the present invention, a junction transistor isdescribed. The junction transistor includes an emitter electrode and abase electrode, which is spaced apart from the emitter electrode suchthat a given voltage can be applied across the emitter and baseelectrodes and, consequently, electrons are emitted by the emitterelectrode toward the base electrode. The junction transistor alsoincludes a first tunneling structure disposed between the emitter andbase electrodes. The first tunneling structure is configured to serve asa transport of electrons between the emitter and base electrodes andincludes at least a first layer of an amorphous material configured suchthat the transport of electrons includes, at least in part, transport bymeans of resonant tunneling with a tunneling probability. The tunnelingprobability depends on the given voltage. The junction transistorfurther includes a collector electrode, which is spaced apart from thebase electrode, and a second tunneling structure, which is disposedbetween the base and collector electrodes. The second tunnelingstructure is configured to serve as a transport, between the base andcollector electrodes, of at least a portion of the electrons emitted bythe emitter electrode such that the portion of electrons is collected bythe collector electrode with a collection efficiency and the transportof electrons includes, at least in part, transport by means oftunneling.

In a seventh aspect of the present invention, an optoelectronicamplification element is described. The optoelectronic amplificationelement is formed by combining the aforementioned field effecttransistor or junction transistor with a detector coupled to the controlelectrode and an emitter coupled to an output. The optoelectronicamplification element is configured such that electromagnetic radiationincident upon the detector generates a voltage across the detector andsubsequently across the control electrodes of the transistor, base andemitter in the case of the junction transistor. The control voltageacross the control electrodes of the transistor in turn controls thebias voltage on the emitting device which may be tuned to emitsubstantially more electromagnetic radiation than the amount initiallyincident upon the device.

In an eighth aspect of the present invention, an optoelectronic mixerelement for simultaneously receiving at least two distinct frequenciesof electromagnetic radiation and producing an output signal having abeat frequency, which beat frequency is a combination of said distinctfrequencies is described. The optoelectronic mixer element includesfirst and second non-insulating layers spaced apart from one anothersuch that a given voltage can be applied across the first and secondnon-insulating layers. The first and second non-insulating layers areconfigured to form an antenna structure for receiving electromagneticradiation of the distinct frequencies. The optoelectronic mixer elementfurther includes an arrangement disposed between the first and secondnon-insulating layers and configured to serve as a transport ofelectrons between the first and second non-insulating layers as a resultof the two distinct frequencies of electromagnetic radiation beingreceived at the antenna structure. The arrangement includes at least afirst layer of an amorphous material such that the transport ofelectrons includes, at least in part, transport by means of resonanttunneling, and such that at least a portion of the electromagneticradiation incident on the antenna is converted to the output signalhaving the beat frequency.

In a ninth aspect of the present invention, an electron tunneling deviceincludes first and second non-insulating layers. The first and secondnon-insulating layers are spaced apart from one another such that agiven voltage can be applied across the first and second non-insulatinglayers, and the first non-insulating layer is formed of a semiconductormaterial while the second non-insulating layer is formed of a metal. Theelectron tunneling device further includes an arrangement disposedbetween the first and second non-insulating layers and configured toserve as a transport of electrons between the first and secondnon-insulating layers. This arrangement includes a first layer of anamorphous material such that using only the first layer of amorphousmaterial in the arrangement would result in a given degree ofnonlinearity in the transport of electrons, with respect to the givenvoltage. However, in accordance with a first aspect of the presentinvention, the arrangement includes a second layer of material, whichsecond layer is configured to cooperate with the first layer ofamorphous material such that the transport of electrons includes, atleast in part, transport by tunneling, and such that the nonlinearity inthe transport of electrons, with respect to the given voltage, isincreased over and above the given degree of nonlinearity.

In a tenth aspect of the invention, the first non-insulating layer inthe electron tunneling device is formed of a superconductor. The firstnon-insulating layer in the electron tunneling device can also be formedof a semi-metal or be in a form of a quantum well or a superlattice.

In an eleventh aspect of the invention, the arrangement in the electrontunneling device further includes a third layer of material, which isconfigured to cooperate with the first layer of the amorphous materialand the second layer of material such that the nonlinearity in thetransport of electrons, with respect to the given voltage, is furtherincreased over and above the given degree of nonlinearity.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood by reference to the followingdetailed description taken in conjunction with the drawings brieflydescribed below.

FIGS. 1A-1E are diagrammatic illustrations of high speed, narrowbanddetectors and a detector array designed in accordance with the presentinvention.

FIGS. 2A-2G are diagrammatic illustrations of high speed emitters and anemitter array designed in accordance with the present invention.

FIGS. 3A and 3B are diagrammatic illustrations of high speed modulatorsdesigned in accordance with the present invention.

FIG. 4 is a diagrammatic illustration of a field effect transistordesigned in accordance with the present invention.

FIG. 5 is a diagrammatic illustration of a junction transistor designedin accordance with the present invention.

FIG. 6 is a diagrammatic illustration of an optoelectronic amplificationelement designed in accordance with the present invention.

FIG. 7A is a diagrammatic illustration of a high speed optoelectronicdevice including one semiconductor layer and one metal layer designed inaccordance with the present invention.

FIGS. 7B-7D are graphs illustrating the schematic conduction bandprofiles of the high speed optoelectronic device of FIG. 7A for variousvoltages applied across the high speed optoelectronic device.

FIG. 7E is a graph of a typical current-voltage curve for the high speedoptoelectronic device of FIG. 7A.

FIG. 8 is a diagrammatic illustration of another high speedoptoelectronic device including one superconductor layer and one metallayer designed in accordance with the present invention.

FIG. 9 is a diagrammatic illustration of still another high speedoptoelectronic device including one quantum well layer and one metallayer designed in accordance with the present invention.

FIGS. 10A and 10B are diagrammatic illustrations of high speedoptoelectronic devices shown here to illustrate possible positions of athird layer in the arrangement positioned between the non-insulatinglayers.

FIG. 11 is a diagrammatic illustration of a high speed optoelectronicdevice including a fourth layer designed in accordance with the presentinvention.

FIG. 12 is a diagrammatic illustration of a high speed optoelectronicdevice including a superlattice structure designed in accordance withthe present invention.

DETAILED DESCRIPTION

Still another alternative to the M-I-M, M-I-M-I-M-I-M and MIS devices isdisclosed in copending U.S. patent application Ser. No. 09/860,988Attorney Docket Number Phiar-P001 of Eliasson and Moddel (Eliasson),which is filed contemporaneously herewith and is incorporated herein byreference. Eliasson discloses an electron tunneling device based onmetal-oxide combinations similar to an M-I-M structure but includingadditional layer of an insulator positioned between the non-insulatinglayers. The additional layer of Eliasson is configured to increase thenonlinearity, asymmetry and differential resistance exhibited by theresulting device over and above the values of these parameters exhibitedby a simple M-I-M device, which does not include the additional layer.The concept of tailoring the layering of the non-insulating andinsulating layers within the electron tunneling device can be extendedto achieve heretofore unseen optoelectronic devices such as narrowbanddetectors, emitters, modulators and transistors, including electrontunneling devices. The optoelectronic devices of the present inventiondiffer from the aforementioned electron tunneling devices of the priorart in that the devices of the present invention are characterized byresonance in the tunneling mechanism while using at least one layer ofan amorphous material. The optoelectronic devices of the presentinvention are illustrated in FIGS. 1A-6 and described immediatelyhereinafter.

Turning now to the figures, wherein like reference numbers are used torefer to like components, attention is immediately directed to FIGS.1A-1E, which illustrate schemes for achieving a narrowband detectorbased on the electron tunneling device. FIG. 1A shows a detector 10A,which includes a first non-insulating layer 12 and a secondnon-insulating layer 14, in addition to first and second insulatinglayers 16 and 18, respectively. First non-insulating layer 12, secondnon-insulating layer 14, first insulating layer 16 and second insulatinglayer 18 combine to form a diode structure 15. Detector 10A alsoincludes antennae 20 and 21. Antennae 20 and 21 can be integrally formedfrom first and second non-insulating layers 12 and 14, respectively, orattached separately thereto. Antennae 20 and 21 are configured to absorbincident electromagnetic energy (not shown) over a specific, narrowrange of wavelengths. Detector 10A further includes a voltage source 22for tuning the characteristics of diode structure 15. In this way,detector 10A achieves a narrow reception bandwidth through the use ofantennae 20 and 21that absorb energy over a narrow range of wavelengths.

FIG. 1B is an edge view of a narrowband detector 10B of the presentinvention, shown here to illustrate an embodiment in which diodestructure 15B is formed of antennae 20B and 21B as first and secondnon-insulating layers with first and second insulating layers 16 and 18disposed therebetween. When electromagnetic radiation 24 is incident onnarrowband detector 10B, electromagnetic radiation 24 is received byantennae 20B and 21B. Antennae 20B and 21B are also configured toreceive input voltage from voltage source 22 such that thecharacteristics of diode structure 15B is tunable.

Referring now to FIGS. 1C and 1D in conjunction with FIGS. 1A and 1B,parallel and series RLC circuits 10C and 10D, respectively, equivalentto the narrowband detector of FIG. 1A or 1B are shown. In parallel RLCcircuit 10C, a box 15C indicates the collection of parallel componentsequivalent to diode structure 15 or 15B. Inside box 15C, a diode 30, aresistor 32, an inductor 34 and a capacitor 36 are arranged in parallelas the equivalent circuit representing diode structure 15 or 15B.Voltage source 22 is shown parallel to box 15C, to be consistent withFIGS. 1A and 1B. A box 24C surrounds the components representing theincident radiation (EM source 40) and the antennae (resistor 42).Similarly, in FIG. 1D, serial RLC circuit 10D includes a box 15Dindicating the collection of series components equivalent to diodestructure 15 or 15B from FIG. 1A or 1B, respectively. A diode 30′, aresistor 32′, an inductor 34′ and a capacitor 36′ are positioned inseries inside box 15D to represent an equivalent circuit for diodestructure 15 or 15B. Again, an EM source 40′ and a resistor 42′ withinbox 24D represent the incident radiation and the antennae, respectively,and a voltage source 22′ is shown to provide a voltage across diode 30′.

Continuing to refer to FIGS. 1C and 1D, in each case, the equivalent RLCcircuit represents components that can be configured into a resonantcircuit which responds optimally only to a limited range of frequencies.The component values can be modified, as known to those skilled in theart, to provide response over a desired range of frequencies. For aparallel RLC circuit having a differential resistance ΔR, theoscillation frequency is given by the equation: $\begin{matrix}{\omega = \lbrack {\frac{1}{LC} - \frac{1}{( {2( {\Delta\quad R} )C} )^{2}}} \rbrack^{1/2}} & (1)\end{matrix}$For a series RLC circuit having a differential resistance ΔR, theoscillation frequency is given by the equation: $\begin{matrix}{\omega = \lbrack {\frac{1}{LC} - \frac{( {\Delta\quad R} )^{2}}{4L^{2}}} \rbrack^{1/2}} & (2)\end{matrix}$In other words, the RLC circuit works to limit the receiving bandwidthof the detector. This effect is important in limiting the noisebandwidth of the detectors of the present invention.

In particular, for a parallel RLC circuit operating in a region where ΔRis positive, the oscillation frequency ω is as given by Equation 1above. For positive values of ΔR, the oscillation is dampened by anexponential factor exp(−1/(ΔR C)), thus yielding a detection bandwidthof 1/(ΔR C) with quality factor$Q = {\frac{1}{\Delta\quad R}{( \frac{L}{C} )^{1/2}.}}$

Similarly, for a series RLC circuit operating in a region where ΔR ispositive, the oscillation frequency ω is as given by Equation 2 above.For positive values of differential resistance ΔR, the oscillation isdampened by an exponential factor exp(−ΔR/2L), thus yielding a detectionbandwidth of ΔR/L with quality factor Q=ΔR(C/L)^(1/2).

The receiving frequency of the narrowband detector of FIG. 1A or 1B isdetermined by Equation 1 or 2, depending on the physical device. FromEquation 1, for small values of L, the 1/LC term again dominates. FromEquation 2, for small values of C, the 1/LC term dominates indetermining the allowed oscillation frequencies. Changing the ΔR valueprovides fine adjustments to the allowed oscillation frequency, andhence the sensitivity off the detector. The fine tuning of the actualdetector device of the present invention should take into account boththe series and parallel RLC tuning circuit analyses to determine theactual allowed oscillation frequencies.

If the detector is not configured as a resonant circuit, a limitation toits frequency response is its ΔRC time constant. The capacitance C canbe reduced, for example, by modifying the geometry of the device. Tofurther decrease the effective ΔRC value, the antenna can be configuredto add an inductive reactance to the diode, compensating the capacitivereactance. Alternatively, the inductive component may be addedphysically in parallel with the diode.

FIG. 1E shows a detector array 50 designed in accordance with thepresent invention. Detector array 50 includes a plurality of detectors10E, each of which detectors 10E has a narrowband receptioncharacteristic as described for detector 10A of FIG. 1A (Voltage source22 of each detector 10A is not shown for simplicity). For example, eachdetector 10E can be configured as detector 10B of FIG. 1B, in which theantennae are integrally formed from the first and second non-insulatinglayers forming diode structure 15B. Furthermore, at least some ofdetectors 10E in detector array 50 can be designed to receive differentfrequencies of input radiation such that detector array 50 is capable ofdetecting narrowband radiation over a range of frequencies. Stillfurther, detector array 50 can be configured such that it is capable ofdetecting radiation over an area larger than the area covered byindividual detectors 10E.

The narrowband detector of FIG. 1A can be modified in a number of ways.For example, the antennae of the narrowband detector can be configuredto simultaneously receive two or more distinct frequencies of incidentelectromagnetic radiation. The diode structure of the narrowbanddetector can be further designed to produce an output electrical signalhaving a beat frequency, which is the difference between the twoincident frequencies. One frequency from a local oscillator may beapplied to the antenna, either in the form of electromagnetic radiationor in the form of an applied electrical signal, and a second frequencycan be an incident electromagnetic radiation. In this way, thenarrowband detector can then be used for heterodyne detection capable ofdetecting high frequency signals that are faster than the detectioncapabilities of the electronics through analysis of the beat frequencyof the output electrical signal.

Turning now to FIGS. 2A-2G, high speed emitters based on the modifiedelectron tunneling device of the present invention are illustrated. FIG.2A shows an emitter 100A, which includes a first non-insulating layer112 and a second non-insulating layer 114. A multilayer tunnelingstructure 116 is disposed between first and second non-insulating layers112 and 114. First non-insulating layer 112, second non-insulating layer114, and multilayer tunneling structure 116 combine to form a diodestructure 115, which directly emits electromagnetic radiation 126. Thisemission of electromagnetic radiation 126 results from the relaxation ofhot electrons (not shown) within diode structure 115 produced directlyfrom an applied voltage from voltage source 122, which includes anintrinsic source impedance (not shown).

Multilayer tunneling structure 116 shown in FIG. 2A can be any suitablestructure which results in diode structure 115 being capable ofproducing electron transport such as, for example, the double insulatorstructure shown in diode structure 15 shown in FIG. 1A. Other multilayertunneling structures can also be incorporated into emitter 100A withoutdeviating from the spirit of the present invention. For example, a thirdinsulator layer can be added to the double insulator structure to forman M-I-I-I-M structure. With the selection of a suitable insulator, aquantum well can formed at both of the insulator-insulator interfaceswith the application of a proper applied voltage. The addition of thisthird insulator layer limits the range of applied voltage values whichyield appreciable current flow, thus resulting in increased nonlinearityin the current-voltage characteristics of the electron tunneling device.As another example, an additional non-insulating layer as well as aninsulator layer such that the device forms the structure M-I-I-M-I-M.The central metal layer can be configured to form a quantum well. Thisquantum well restricts the energy levels at which electrons can traversethe M-I-I-M-I-M structure, thus resulting increased asymmetry at anoptimal value of applied voltage. As still another example, themultilayer structure can be of the form I-M-I-I-M-I-M-M-I-M-I-I suchthat the overall structure takes the form M-I-M-I-I-M-I-M-M-I-M-I-I-M.In this case, the multilayer produces a specific range of allowedenergies for electrons tunneling through the structure which is broaderthan the range of energies allowed by the inclusion of a single isolatedquantum well. In this way, the range of voltages at which resonanttunneling takes place can be tailored for a desired application.

FIG. 2B illustrates another emitter 100B designed in accordance with thepresent invention. Emitter 100B is similar in structure to emitter 100Aof FIG. 2A but further includes antennae 120 and 121. In the case ofemitter 100B, the applied voltage from voltage source 122, or appliedcurrent (not shown), biases diode structure 115 in a region of negativedifferential resistance, resulting in electronic resonance effects. Thisresonance causes the electron transport between the antennae tooscillate rapidly, thus resulting in the emission of electromagneticradiation from the antennae.

FIG. 2C is an edge-view of an emitter 100C of the present invention,shown here to illustrate an embodiment in which diode structure 115C isformed of antennae 120C and 121C as first and second non-insulatinglayers with multilayer tunneling structure 116 disposed therebetween.When a voltage is applied across diode structure 115C by voltage source122 such that the diode structure is biased in a region of negativedifferential resistance, emitter 100C emits electromagnetic radiationfrom the antennae due to electronic resonance.

Referring now to FIGS. 2D and 2E in conjunction with FIG. 2B, paralleland series RLC circuits 100D and 100E, respectively, equivalent to theemitter of FIG. 2B are shown. In parallel RLC circuit 100D, a box 115Dindicates the collection of parallel components equivalent to diodestructure 115. Inside box 115D, a diode 130, a resistor 132, inductor134 and a capacitor 136 are arranged in parallel as the equivalentcircuit representing diode structure 115 of FIG. 2B. Voltage source 122is positioned parallel to box 15C, to be consistent with FIG. 2B. Aradiating antenna 138 represents antennae 120 and 121. Similarly, inFIG. 2E, serial RLC circuit 100E includes a box 115E indicating thecollection of series components equivalent to diode structure 115 ofFIG. 2B. A diode 130′, a resistor 132′, an inductor 134′ and a capacitor136′ are positioned in series inside box 115E to represent an equivalentcircuit for diode structure 115 with a radiating antenna 138′ and avoltage source 122′ to provide a voltage across diode 130′.

Continuing to refer to FIGS. 2D and 2E, in each case, the equivalent RLCcircuit represents components that can be configured into a resonantcircuit which oscillates optimally only over a limited range offrequencies. The component values can be modified, as known to thoseskilled in the art, to provide response over a desired range offrequencies.

Still referring to FIGS. 2D and 2E, it is noted that the aforedescribedemitters take advantage of the negative differential resistanceexhibited by the electron tunneling device of the present invention. Bybiasing the emitter in the negative differential resistance region, thevoltage across the insulating layers of the emitter oscillates at anoscillation frequency. Subsequently, the output antenna radiates at theoscillation frequency. By ensuring that the oscillation is uniform,coherent radiation is achieved. Also, by controlling the oscillationfrequency by controlling the voltage bias and by modifying the devicedimensions, the frequency of the radiation can be controlled. In thisway, the emitters of the present invention can potentially replacecurrently available optical sources, such as lasers. The radiationemitted by the emitters of the present invention can be readily tailoredfrom the visible frequencies into the far-infrared by scaling the sizeof the device. Due to the lower resistivity of the metal-oxidecombination devices in comparison to semiconductor devices, the emittersof the present invention can be made to operate at higher frequenciesthan semiconductor oscillators.

The emitter oscillation frequency can be controlled, for example, byintegrating an RLC circuit into the emitter. The conditions of theoscillation frequency control can be analyzed by considering the emitterin a series RLC circuit or a parallel RLC circuit, as discussed infurther detail immediately hereinafter.

For a parallel RLC circuit having a differential resistance ΔR, theoscillation frequency is given by the equation: $\begin{matrix}{\omega = \lbrack {\frac{1}{LC} - \frac{1}{( {2( {\Delta\quad R} )C} )^{2}}} \rbrack^{1/2}} & (3)\end{matrix}$

Several conclusions can be drawn from Equation 3:

-   -   1. To achieve high oscillation frequency, the inductance L must        be small;    -   2. For high oscillation frequency, |ΔR| must be large;    -   3. For high oscillation frequency, given small L and large ΔR,        the value of C must be optimized.    -   4. For sufficiently small values of C, the (1/LC) term will        dominate, and, within certain limits, the magnitude of |ΔR| will        have only a small effect on the oscillation frequency ω.

For a series RLC circuit having a differential resistance ΔR, theoscillation frequency is given by the equation: $\begin{matrix}{\omega = \lbrack {\frac{1}{LC} - \frac{( {\Delta\quad R} )^{2}}{4L^{2}}} \rbrack^{1/2}} & (4)\end{matrix}$Again, several conclusions can be drawn from Equation 4:

-   -   1. To achieve high oscillation frequency, the capacitance C must        be small;    -   2. For high oscillation frequency, |ΔR| must be close to zero—in        other words, the negative differential region of the I-V curve        should be as nearly vertical as possible;    -   3. For high oscillation frequency, given small C and small ΔR,        the value of L must be optimized;    -   4. For sufficiently small values of C, the (1/LC) term will        dominate, and, within certain limits, the magnitude of |ΔR| will        have only a small effect on the oscillation frequency ω.

The experimental devices so far fabricated in accordance with thepresent invention may be best modeled either as parallel or as seriesRLC circuits. Thus one of the two analyses may be used in optimizing theoscillation frequency of the emitter. In either case, the RLC circuitryacts effectively as integration of a tuning circuit to control theoscillation frequency of the emitter.

The stability of the oscillation frequency can be achieved, forinstance, by integrating the emitter into a structure havingelectromagnetic feedback, such as a Fabry-Perot cavity, or coupled to adistributed Bragg reflector. The reflected electromagnetic energyprovides a frequency-dependent feedback voltage that stabilizes theoscillation frequency. The mechanism of the oscillation frequencystabilization using this method is similar to stimulated emission from again medium in a laser. Stabilization of the oscillation frequency inthe electron tunneling device of the present invention results from thefact that the electron tunneling device can operate both as an emitterand a receiver. The reflected electromagnetic energy is reabsorbed inthe electron tunneling device to cause a slight shift in the operatingvoltage. The shift in the operating voltage changes the value of thedifferential resistance exhibited by the device, thus causing a shift inthe oscillation frequency. In this way, the feedback mechanism works tostabilize the oscillation frequency of the device.

A possible implementation of the aforedescribed feedback mechanism isillustrated in FIG. 2F. FIG. 2F shows an emitter 100F coupled to anoptical fiber 140 having an integrated distributed Bragg reflector(DBR), indicated by box 142. DBR 142 is configured to reflectelectromagnetic radiation of a particular wavelength, which is partiallyreabsorbed by the emitter and stabilizes the emission wavelength (i.e.,frequency) to a desired value. Thus, the oscillation frequency ofemitter 100F is stabilized.

FIG. 2G shows an emitter array 150 designed in accordance with thepresent invention. Emitter array 150 includes a plurality of emitters100G, each of which emitters 100G having a narrowband emissioncharacteristic as described for emitter 100B of FIG. 2B (Voltage source122 of each emitter 100B is not shown for simplicity). For example, eachemitter 100G can be configured as detector 100C of FIG. 2C, in which theantennae are integrally formed from the first and second non-insulatinglayers forming diode structure 115C. Furthermore, at least some ofemitters 100G in emitter array 150 can be designed to emit differentfrequencies of radiation such that emitter array 150 is capable ofproducing narrowband radiation over a range of frequencies. Stillfurther, emitter array 150 can be configured such that it is capable ofemitting radiation over an area larger than the area covered byindividual emitters 100G. Moreover, emitter array 150 can be configuredwith feedback between elements to provide coherent emission across thearray.

Returning briefly to FIG. 1A, a possible modification of narrowbanddetector 10A is to configure antennae 20 and 21 to reflect or absorb theincident electromagnetic energy according to the value of dynamicresistance exhibited by the detector, where the dynamic resistance isinversely proportional to the slope of the I-V curve of the device.Noting that the dynamic resistance exhibited by narrowband detector 10Ais a strong function of bias voltage (as shown in FIG. 7E) applied tothe device by voltage source 22, it is readily apparent that theincident electromagnetic energy is reflected when the dynamic resistanceis very low or very high, and, at intermediate values of dynamicresistance, when the dynamic resistance is matched to the antennaimpedance, the electromagnetic energy is absorbed. The absorbedelectromagnetic energy is converted to DC electrical power due to theelectrical rectification effect of the narrowband detector. Since theelectromagnetic energy absorption/reflection can be modulated bychanging the applied bias voltage, the narrowband detector of thepresent invention can function as a modulator by monitoring theelectromagnetic energy reflected by the device. Since the narrowbanddetector of the present invention operates at the frequency of theincident electromagnetic energy, the modulation rate is limited only bythe rate at which the bias voltage can be applied to the device. Therate of applied bias voltage is a function of the RC time constant ofthe device and the signal transmission time, and the RC time constant inthe devices of the present invention can be made very small since thecapacitance of each of the devices can be made very small by virtue ofits dimensions. To further decrease the effective RC value, the antennacan be configured to add an inductive reactance to the diode,compensating the capacitive reactance. Alternatively, the inductivecomponent may be added physically in parallel with the diode.

The antenna structure can also be tailored to absorb 70% or more of theincident electromagnetic energy (see, for example, Z. B. Popovic, etal., “A low-profile broadband antenna for wireless communications,”Sixth IEEE International Symposium on Personal, Indoor and Mobile RadioCommunications (PIMRC '95) Wireless: Merging onto the InformationSuperhighway (Cat. No. 95TH8135), vol. (xxvii+xxiii+1389), 135-9,vol. 1. Using such an antenna structure, the modulator of the presentinvention can be made to switch between up to 70% and 0% absorption toachieve a contrast ratio of 2.3 or better. Such a contrast ratio valueis acceptable for various applications such as electromagnetic signalmodulation. Such modulators may be combined in series to provide greaterdepth of modulation.

Furthermore, the dimensions of the antenna structure and the electrontunneling device area can be made to range from sub-microns up tohundreds of microns and more, with the result that the wavelength atwhich the modulation occurs can be selected over a large range.Therefore, by modifying the antenna design and scaling the antennadimensions, the modulator of the present invention can be made tofunction over a wide range of wavelengths. In addition, the array ofdetectors shown in FIG. 1E can be made to function as an array ofmodulators by proper antenna design. Since the amount of voltagerequired to bias each modulator in a region of high absorption rangesfrom a fraction of a volt up to just a few volts, arrays of modulatorscan be coupled, for example, to a charge-coupled device. In this way,the modulator array can function as a spatial light modulator.

Additional modifications to the modulator can be made. For example, asecond antenna can be added to each modulator such that the secondantenna, perhaps tuned to a different polarization or wavelength, can beoptically addressed to be set to reflect or absorb incidentelectromagnetic energy. Also, by operating the device in the region ofnegative differential resistance (i.e., the negative slope region of theresonance peak), a substantial gain can be added to the dc voltagegenerated in the electronic rectification process. Thus, the modulatorsof the present invention exhibit extremely fast response, broad regionof operation extending from the visible to the far-infrared, lowoperating voltage and an adequate modulation ratio.

FIGS. 3A and 3B are illustrations of the aforedescribed modulators ofthe present invention. FIG. 3A illustrates a broadband modulator 200Aincluding broadband, receiving antennae 220A and 221A as well asbroadband, modulating antennae 222A and 223A. Receiving antennae 220Aand 221A detect a first electromagnetic radiation 201A incident thereonand produces a voltage across a diode structure 210A. Diode structure210A can be, for example, diode structure 15 shown in FIG. 1A or diodestructure 115 as shown in FIG. 2A or 2B. The induced voltage acrossdiode structure 210A subsequently alters the resistance, capacitance, orresponsivity of the diode structure, which in turn modifies theabsorption characteristics of modulating antennae 222A and 223A. As aresult, modulation antennae 222A and 223A act to modulate a secondelectromagnetic radiation 202A, thus re-emitting second electromagneticradiation 202A toward an output (not shown) as an output electromagneticradiation 203A. Consequently, first electromagnetic radiation 201 Areceived at the receiving antenna acts to modulate secondelectromagnetic radiation 202A which interacts with the modulatingantenna to re-emit output electromagnetic radiation 203A.

The detecting and modulating antennas of a modulator of the presentinvention can be broadband, as shown in FIG. 3A, narrowband, or acombination thereof. FIG. 3B illustrates the use of narrowband antennaefor both emitting and receiving antennae. A narrowband modulator 200Bincludes a diode structure 210B, narrowband, receiving antennae 220B and221B as well as narrowband, modulating antennae 222B and 223B. Diodestructure 210B can again be in the form, for example, of diode structure15 shown in FIG. 1A or diode structure 115 of FIG. 2A or 2B. Narrowbandmodulator 200B operates in essentially the same way as broadbandmodulator 200A of FIG. 3A with a difference in that, since the receivingand modulating antennae are configured to function over a narrow rangeof frequencies, narrowband modulator 200B works as a modulator only inthe narrow range of frequencies. Such a narrowband modulator is usefulin certain applications such as optical communications. The modulator ofthe present invention is configurable to operate as a digital device, inwhich the receiving antennae produces only discrete, voltage valuesacross the diode structure. Alternatively, the modulator is alsoconfigurable as an analog device, in which the receiving antennae isconfigured to cooperate with the diode structure to produce continuousvalues of voltages such that the absorption characteristics of themodulating antennae can be modulated over a continuum of absorptivityvalues.

Referring now to FIG. 4, a field effect transistor 300 designed inaccordance with the present invention is illustrated. Field effecttransistor 300 includes a substrate 310, on which other components aredeposited. Field effect transistor 300 further includes a sourceelectrode 312 and a drain electrode 314 with first and second insulatinglayers 316 and 318, respectively, disposed therebetween. First andsecond insulating layers 316 and 318 can be replaced, for example, by amultilayer tunneling structure, such as multilayer tunneling structure116 as described in the context of FIG. 2A. A passivation layer 320separates the source and drain electrodes and the first and secondinsulating layers from a gate electrode 322, which is configured toallow the application of a voltage to modulate the potential (not shown)in first and second insulating layers 316 and 318. The modulation of theelectric field within the first and second insulating layers is a resultof the change in the tunneling probability of electrons flowing throughthe multilayer oxide between the source and drain electrodes due to theapplication of the voltage to the gate electrode. Unlike semiconductortransistors, field effect transistor 300 is based on the mechanism ofelectron tunneling, rather than electron and hole transport in asemiconductor band. Therefore, field effect transistor 300 can operateat much higher frequencies than the presently available semiconductortransistors.

Turning to FIG. 5, a junction transistor 400 designed in accordance withthe present invention is illustrated. Junction transistor 400 includesan emitter electrode 412, a base electrode 414 and a collector electrode416. A first multilayer tunneling structure 418 is disposed between theemitter and base electrodes. A second multilayer tunneling structure 420is disposed between the base and collector electrodes. As known to oneskilled in the art, junction transistors use bias voltages or currentsfrom an external bias source (not shown) to set the operating point ofthe transistor, and power to drive the output. These external biassources are configured to apply voltage, for example, in a commonemitter configuration, as a potential at the base-emitter junctionand/or as a potential at the collector-emitter junction. For instance, abias source can be used to apply a voltage across the emitter and baseelectrodes to control the potential in first multilayer tunnelingstructure 418 and, consequently, the tunneling probability of electronsfrom emitter electrode 412 to base electrode 414. Once emitted,electrons tunnel through first multilayer tunneling structure 418, baseelectrode 414, second multilayer tunneling structure 420 and finallyinto collector electrode 416 with a given value of collectionefficiency. The collection efficiency is a function of the fraction ofelectrons that tunnel unimpeded through the base. The tunnelingprobability is determined by the applied voltage to the base, along withother material properties. Again, unlike semiconductor transistors,junction transistor 400 is based on the mechanism of electron tunneling,rather than electron and hole transport in a semiconductor band.Therefore, junction transistor 400 can operate at much higherfrequencies than the presently available semiconductor transistors.

A number of the optoelectronic devices described above can be combinedto form other optoelectronic components. For example, in FIG. 6, anoptoelectronic amplification element 500 is described. Optoelectronicamplification element 500 includes a transistor element, represented bya box 510. Suitable transistor element for use in optoelectronicamplification element 500 are, for example, the aforementioned fieldeffect transistor of FIG. 4 and the junction transistor of FIG. 5.Optoelectronic amplification element further includes a detector 512,which is coupled to a control electrode (the gate or base electrode,depending on the transistor type) of the transistor, and an emitter 514for generating electromagnetic radiation 518. In optoelectronicamplification element 500 shown in FIG. 6, electromagnetic radiation 516incident upon the detector generates a voltage at the detector and,subsequently, across the control electrodes of the transistor (forexample, across the base and emitter electrodes for a junctiontransistor). As known to one skilled in the art, transistors use powerfrom an external bias source (not shown) to drive an output, here shownas emitter 514. In this way, the generated voltage across the controlelectrodes of the transistor in turn alters the bias voltage on emitter514, which can be tuned to emit substantially more electromagneticradiation than the amount initially incident upon the device.

Turning now to FIGS. 7A-7E, possible modifications to the high speedoptoelectronic devices of the present invention are described. Theco-pending application of Eliasson discloses the possibility of using adifferent metal to form each of the non-insulating layers as well asdifferent amorphous insulators as the first insulating layer and thesecond layer of material. However, although the electron tunnelingdevice disclosed in Eliasson is suitable for solar energy conversionapplications, it is possible to further enhance the flexibility of theEliasson device by incorporating materials other than metals into theelectron tunneling device of Eliasson. Furthermore, the devices of thepresent invention are especially distinguishable from the electrontunneling devices of the prior art, such as the M-I-M, M-I-M-I-M-I-M andS-I-I-S devices, by the use of resonant tunneling using at least onelayer of an amorphous material in the devices. Several suchmodifications are discussed immediately hereinafter.

FIG. 7A illustrates a high speed, optoelectronic device 600. LikeEliasson, optoelectronic device 600 includes first and secondnon-insulating layers 612 and 14, respectively, with a first insulatinglayer 16 of an amorphous material and a second layer 18 of a materialpositioned therebetween. First non-insulating layer 612 and secondnon-insulating layer 14 are spaced apart such that a given voltage canbe provided therebetween. The given voltage can be, for instance, a biasvoltage from an external voltage source (not shown) that is directlyapplied to the first and second non-insulating layers. First insulatinglayer 16 can be, for example, a native oxide of first non-insulatinglayer 612. For instance, if a layer of chromium is used as firstnon-insulating layer 612, the layer of chromium can be oxidized to forma layer of chromium oxide to serve as first insulating layer 16. Othersuitable materials include, but are not limited to, silicon dioxide,niobium oxide, titanium oxide, aluminum oxide, zirconium oxide, tantalumoxide, hafnium oxide, yttrium oxide, magnesium oxide, silicon nitrideand aluminum nitride. In contrast to Eliasson, first non-insulatinglayer 612 of optoelectronic device 600 is formed of a semiconductorwhile second non-insulating layer 14 is formed of a metal.Optoelectronic device 600 is useful in applications such as detectors,in which specific characteristics of the semiconductor positivelycontribute to the detector efficiency. In addition, optoelectronicdevice 600 generally exhibits the high nonlinearity and asymmetry in theelectron conduction, including resonant tunneling, as demonstrated inthe Eliasson device, without the complicated structure of theM-I-M-I-M-I-M device of Suemasu. As an alternative, a semi-metal can beused as first non-insulating layer 612 rather than a semiconductor. Asemiconductor in place of one of the metal electrodes allows furthertailoring of the diode performance by introducing a band gap in thedensity of states in one electrode. The density of states, for example,permits enhanced negative differential resistance, resulting from theprovided voltage shifting the source of tunneling of electrons throughresonant energy levels from regions of high electron concentration(conduction band) to regions of no electron concentration (bandgap). Asemiconductor provides electrons that are more confined in energy thandoes a metal, and therefore the negative differential resistance isenhanced. Furthermore, a semimetal, in place of one of the metalelectrodes, provides yet another source of electrons within a confinedenergy range for the emitting electrode.

Continuing to refer to FIG. 7A, second layer 18 is formed of apredetermined material, which is different from first insulating layer16 and is configured to cooperate with first insulating layer 16 suchthat first insulating layer and second layer 18 serve as a transport ofelectrons between the first and second non-insulating layers. Thepredetermined material, which forms second layer 18, can be, forexample, an amorphous insulator such as, but not limited to, chromiumoxide, silicon dioxide, niobium oxide, titanium oxide, aluminum oxide,zirconium oxide, tantalum oxide, hafnium oxide, yttrium oxide, magnesiumoxide, silicon nitride, aluminum nitride and a simple air or vacuum gap.Preferably, second layer 18 is formed of a material having asignificantly lower or higher work function than that of first amorphouslayer such that the device exhibits an asymmetry in the energy banddiagram.

Had the optoelectronic device consisted of only the first and secondnon-insulating layers and the first insulating layer, the device wouldbe essentially equivalent to a prior art M-I-M-based device and wouldexhibit a given degree of nonlinearity, asymmetry and differentialresistance in the transport of electrons. However, the inclusion ofsecond layer 18, surprising and unexpectedly, results in increaseddegrees of nonlinearity and asymmetry over and above the given degree ofnonlinearity and asymmetry while the differential resistance is reduced,with respect to the given voltage. In addition, the use of thesemiconductor or semimetal as the first non-insulating layer 612 furtherenhances the device efficiency. The increase in the nonlinearity andasymmetry and reduction in differential resistance is achieved withoutresorting to the use of epitaxial growth techniques or crystallinelayers of the aforedescribed prior art. The mechanism of this increaseis described immediately hereinafter in reference to FIGS. 7B-7E.

Referring to FIGS. 7B-7D in conjunction with FIG. 7A, a schematic of aenergy band profile 620 corresponding to electron tunneling device 600and the changes in the energy band profile due to voltage applicationare illustrated. Energy band profile 620 includes four regionscorresponding to the four layers of electron tunneling device 600.Energy band profile 620 represents the height of the Fermi level in thenon-insulating layers and the height of the conduction band edge infirst insulating layer 16 and second layer 18 (y-axis 622) as a functionof distance (x-axis 624) through optoelectronic device 600 in theabsence of provided voltage across the device. FIG. 7C illustrates afirst modified energy band profile 630 when a voltage is provided in areverse direction to M-I-M device 600. The voltage may be provided by,for example, an applied external voltage or an induced voltage due tothe incidence of electromagnetic energy. In the case shown in FIG. 7C,the primary mechanism by which electrons travel between the first andsecond non-insulating layers is tunneling in a reverse direction,represented by an arrow 636. In contrast, when a forward bias voltage isprovided, a second modified energy band profile 640 results, as shown inFIG. 7D. In the case of the situation shown in FIG. 7D, tunneling of theelectrons occurs along paths 646 and 646′, but there now exists aquantum well region through which resonant tunneling occurs, as shown byarrow 648. In the region of resonant tunneling, the ease of transport ofelectrons suddenly increase, therefore resulting in increased currentbetween the non-insulating layers of electron tunneling device 600.

Continuing to refer to FIG. 7D, the addition of second layer 18 providesa path for electrons to travel through the device by a resonanttunneling rather than the ordinary tunneling process of the prior artM-I-M device. As a result, more current flows between the non-insulatinglayers of electron tunneling device 600, as compared to the M-I-Mdevice, when a positive voltage is provided while the current flow witha negative voltage provided to the electron tunneling device of thepresent invention. The presence of resonant tunneling in electrontunneling device 600 therefore results in increased nonlinearity andasymmetry with decreased differential resistance in comparison to theprior art M-I-M device.

A typical I-V curve 650 corresponding to electron tunneling device 600is shown in FIG. 7E. I-V curve 650 demonstrates that electron tunnelingdevice 600 functions as a diode, where the diode is defined as atwo-terminal electronic element. Furthermore, I-V curve 650 is shown toinclude a resonance peak 656 corresponding to the provided voltageregion in which resonant tunneling occurs. The appearance of resonanttunneling in actually fabricated devices of the present inventiondepends on the precision of the fabrication process. Even when resonancepeak 656 is not present, I-V curve 650 exhibits a higher degree ofasymmetry and nonlinearity in comparison to the I-V curve of a prior artM-I-M device (for example, as shown in FIG. 1E of Eliasson). In otherwords, while the presence of a resonance peak in the I-V curve of anelectron tunneling device of the present invention may lead toadditional advantages in certain applications, such as greatly increasednonlinearity around the resonance peak, the electron tunneling device ofthe present invention achieves the goal of increased asymmetry andnonlinearity with reduced differential resistance in thecurrent-to-voltage performance even when the averaging effect of theamorphous layer “washes out” the resonance peak. Therefore, electrontunneling device 600 essentially includes all of the advantages of theprior art M-I-M-I-M-I-M device, without the complicated fabricationprocedure and the use of exotic materials, and all of the advantages ofthe prior art SIIS device, without the drawbacks of the use ofsemiconductor materials as described above. Despite and contrary to theteachings of Suemasu, the electron tunneling device of the presentinvention is able to achieve increased nonlinearity and asymmetry anddecreased differential resistance in the transport of electrons throughthe device, using readily available metals and insulators in a simplestructure that is simply manufactured compared to the more complexmanufacturing processes of the prior art.

Referring now to FIGS. 8 and 9 in conjunction with FIG. 7A, morepossibilities for materials are described. An optoelectronic device 700of FIG. 8 includes a modification from the optoelectronic device of FIG.7A in that first non-insulating layer 712 is now formed of asuperconductor. In contrast, an optoelectronic device 800 of FIG. 9includes a first non-insulating layer 812 in a form of a quantum well.The quantum well is contacted appropriately on its edge or through abarrier layer (not shown), as known to those skilled in the art. Theinclusion of the different materials in optoelectronic devices 700 and800 leads to different conduction band diagrams and, hence, differentelectron tunneling properties. Optoelectronic devices 700 and 800 dostill exhibit resonant tunneling with the appropriate applied voltage,but the exact shape of the I-V curve can be tailored according to theapplication by adjusting the material properties of the superconductorand the quantum well, respectively. Superconductor layers enhanceM-I-I-M device by providing non-linearity at extremely low providedvoltages. This is made possible by the relatively small provided voltagerequired to induce the tunneling of quasi-particles. Furthermore,superconductors can provide lower resistance to the device, which canreduce resistance losses.

In the optoelectronic devices shown in FIGS. 7A-9, it is also possibleto have a current flow by the mechanism of hot electron tunneling, inwhich no external voltage is needed. In hot electron tunneling,electrons tunnel from the first non-insulating layer to the secondnon-insulating layer, and vice versa, without the application of avoltage across the non-insulating layers. This process is useful inapplications where antennas are not used to couple the electric fieldacross the oxide of the diode and the electromagnetic radiation exciteselectrons directly within the electrodes, and also when the energies ofthe incident photons are substantially larger than the barrier providedby the insulating multilayer tunneling barrier.

Turning now to FIGS. 10A and 10B, optoelectronic devices including athird layer are illustrated. Optoelectronic devices 900A and 900B eachincludes a first and second non-insulating layers 912 and 14,respectively, with first and second insulating layers 16 and 18,respectively, positioned therebetween. First and second non-insulatinglayers can be a metal, semiconductor, semi-metal, superconductor,quantum well, superlattice or a combination thereof. Furthermore,optoelectronic devices 900A and 900B include third layers 920A and 920B,respectively. Third layer 920A is shown to be located between secondinsulating layer 18 and second non-insulating layer 14 in FIG. 10A,while third layer 920B is shown between first and second insulatinglayers 16 and 18, respectively. In either case, the third layer isconfigured such that the nonlinearity and asymmetry in the I-V curve ofthe respective optoelectronic device are further increased over a devicewith just the first two insulating layers. The presence of the thirdlayer functions to enhance the electron tunneling between the first andsecond non-insulating layers by providing an additional path throughwhich the electrons are transported by resonant tunneling. With theapplication of an appropriate voltage across the first and secondnon-insulating layers of the device including the third layer, two pathsexist through which the electrons can potentially travel by resonanttunneling. By matching the two paths, the electron transport is enhancedin comparison to the optoelectronic device with only one or no resonanttunneling paths, thus increasing the degree of nonlinearity andasymmetry in the I-V curve. In other words, the addition of this thirdlayer further limits the range of applied voltage where appreciablecurrent flows, resulting in increased nonlinearity. The third layer canbe, for example, a third insulator, a third non-insulating layer (suchas a metal, semiconductor, semi-metal, superconductor, superlattice or aquantum well) formed by known techniques such as sputtering or atomiclayer deposition. If the third layer, for example, is a quantum well,the result is discrete allowed energy levels. By matching up the allowedenergy level within the quantum well, by a provided voltage, to thoseproduced within the first and second insulating layers, an increasedtunneling current between the two non-insulating layers is produced. Aswith a semiconductor, a superlattice provides electrons that are highlyconfined in energy, and therefore the negative differential resistanceis enhanced.

Further extending the idea of additional layers between thenon-insulating layers, FIGS. 11 and 12 illustrate optoelectronic devices1000 and 1100, respectively, with further enhanced resonant tunneling bythe addition of a plurality of layers between first and secondnon-insulating layers. Optoelectronic device 1000 of FIG. 11 includes athird non-insulating layer 1020 and a third insulating layer 1022between a first non-insulating layer 1012 and second non-insulatinglayer 14, in addition to first and second insulating layers 16 and 18,respectively. The addition of third non-insulating layer 1020 and thirdinsulating layer 1022 serves to create additional resonance peaks in theI-V curve or to fine tune the location of the original resonance peak inthe I-V curve of optoelectronic device 1000. Taking this idea a stepfurther, optoelectronic device 1100 of FIG. 12 includes a plurality ofadditional, alternating non-insulating layers 1112 and barrier layers1116 cooperating to form a superlattice structure. The superlatticeallows one to perform bandgap engineering and define the energy regionsof highly confined electrons. For example, it is possible to develop aregion with designed bands of electrons and holes and band gaps.Thereby, we are not limited to the material properties of readilyavailable materials.

Although each of the aforedescribed embodiments have been illustratedwith various components having particular respective orientations, itshould be understood that the present invention may take on a variety ofspecific configurations with the various components being located in awide variety of positions and mutual orientations and still remainwithin the spirit and scope of the present invention. Furthermore,suitable equivalents may be used in place of or in addition to thevarious components, the function and use of such substitute oradditional components being held to be familiar to those skilled in theart and are therefore regarded as falling within the scope of thepresent invention. For example, the exact materials used in theaforedescribed devices may be modified while achieving the same resultof high speed optoelectronic devices. Also, the various optoelectronicdevices described above can be combined to form a high speedoptoelectronic integrated circuit, which would have advantages of speed,purely optoelectronic input/output, response from the visible throughthe far-infrared range, scalability and greater ease of fabrication withless toxic materials in comparison to semiconductor integrated circuits.Furthermore, the present invention can also be used as a basis for otherapplications such as, but not limited to, memory, charge-coupled device(CCD), electronically-addressed spatial light modulator (EASLM),optically-addressed spatial light modulator (OASLM), high speedelectronic components, integrated circuits, millimeter wave detector,heat sensor (long wave infrared detectors), frequency converter,multiplexer, demultiplexer and combinations thereof. For example, anoptoelectronic mixer element for simultaneously receiving at least twodistinct frequencies of electromagnetic radiation and producing anoutput signal having a beat frequency, which beat frequency is acombination of said distinct frequencies can be formed. Theoptoelectronic mixer element includes a multilayer structure (such asthe aforedescribed double insulator structure) positioned between a pairof non-insulating layers, which are configured to form an antennastructure for receiving electromagnetic radiation of the two distinctfrequencies. The multilayer structure is configured to serve as atransport of electrons between the pair of non-insulating layers as aresult of the two distinct frequencies of electromagnetic radiationbeing received at the antenna structure. The electron transport occursat least in part by resonant tunneling such that at least a portion ofthe electromagnetic radiation incident on the antenna is converted to anoutput signal having the beat frequency. Such an optoelectronic mixercan be used simply to produce an output beat signal having a beatfrequency such that the output beat signal is a “mix” of the two inputsignals. Mixing allows the detection of the lower beat frequencies ofhigh frequency signals that are faster than the detection capabilitiesof conventional electronic detectors. The element can also be used forheterodyne detection of information encoded on a high frequency carriersignal. The information may be extracted by mixing the high frequencysignal with the output from a local oscillator of the same carrierfrequency. Therefore, the present examples are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein but may be modified within the scope of theappended claims.

1. An electron tunneling device comprising: a first arrangementincluding first and second non-insulating layers spaced apart from oneanother such that a given voltage can be applied across the first andsecond non-insulating layers; and a second arrangement disposed betweenthe first and second non-insulating layers and configured to produceelectron tunneling therethrough, said second arrangement including i) afirst insulating layer such that using only said first insulating layerwould result in a given degree of nonlinearity in the electrontunneling, with respect to said given voltage, and ii) a second,different insulating layer disposed directly adjacent to and configuredto cooperate with said first insulating layer such that saidnonlinearity in the electron tunneling, with respect to said givenvoltage, is enhanced over and above said given degree of nonlinearity bythe inclusion of said second insulating layer.